Hi John,
Other Users have made Plasma Tables using both Mach3 and KMotionCNC. I don't know of exactly how they turned out. KFLOP works well with dual loop control for Z height. The inner loop controls height based on steps (or encoder) and the outer based on Torch Voltage. You can use an MCode to switch between modes on the fly. I would think proportional voltage feedback would work better than the UP/Down feedback that some units operate with.
What kind of changes to KMotionCNC GUI do you think would be required? I wouldn't think you would need to make extensive changes. I don't think any C++ programming at all would be required for basic operation. Our projects are built with VS 2008. To go backwards might be difficult. They use MFC so it is easiest to use VS Standard Version or higher (Free Express Versions are more difficult).
Mach3 has better screen customization capabilities but we feel KMotionCNC has better Trajectory Planning and robustness.
HTH
Regards